This invention relates to a scratch-resistant styreneacrylonitrile copolymer (SAN) article particulary suitable for optical devices such as windows, lenses or mirrors. More particularly, this invention relates to a thin silica based coating which is transparent, strongly adherent to the SAN substrate, weatherable and abrasion resistant.
The optical clarity, low cost and light weight of the styrene-acrylonitrile copolymers have long been recognized. However, their use in many applications has been hindered by their softness which renders them susceptible to scratching.
The general concept of applying an abrasion-resistant, silica or silicate type coating to a soft polymeric substrate is not novel, but each application presents a distinct set of problems. This invention is directed at producing a strongly adherent and abrasion-resistant coating which retains its desired properties after prolonged exposure to temperature and humidity extremes and changes.
Abrasion-resistant coatings, prepared in accordance with the prior art, could not withstand exposure to repeated changes in temperature and humidity. A series of twelve humidity/thermal crack cycles was used as a representative screening test with each cycle consisting of 1) a 24 hour exposure to 100.degree.F. and 100% relative humidity, 2) 20 hours at minus 10.degree.F., and then 3) four hours at ambient room conditions. After exposure to this series of crack cycles, the various coatings were evaluated and none were capable of providing adequate abrasion resistance to the SAN substrate. Two distinct failure modes were evident. First, there were coated samples which were severely crazed by the series of crack cycles, and this, of course, destroyed the optical clarity of the article. This failure was apparently caused by the internal stresses created by the difference between the coefficient of expansion of the coating and that of the substrate. The magnitude of these stresses becomes apparent when one realizes that the coefficients of expansion of a silica-based coating and the substrate may differ by one order of magnitude or more. The other failure mode was a loss of adhesion between the coating and the substrate. If adhesion is lost, the coating can be scuffed off during cleaning and wiping operations, leaving the SAN unprotected from abrasion. Thus, strong adhesion developed through the stable chemical linkages between the coating and the substrate is necessary.
It is an object of my invention to provide a laminated article comprising an abrasion-resistant silicate coating chemically bonded to a styrene-acrylonitrile copolymer substrate, which article is clear, abrasion-resistant and weatherable.
It is a further object of my invention to provide a clear, abrasion-resistant coating, which coating is bonded to a transparent styrene-acrylonitrile copolymer article, and which coating is sufficiently flexible to withstand severe thermal shocks without cracking or crazing.
It is another object of my invention to provide a process for coating the styrene-acrylonitrile copolymer with a clear, abrasion-resistant and weatherable film.